1. Field of the Invention
The present invention relates to an amplifier used at a receiving front end of a digital radio communication apparatus.
It also relates to a single side phase converter circuit for amplifying by single side phase conversion.
2. Description of the Related Art
In mobile communication, since the receiving electric field varies significantly depending on the distance between the base station and a mobile communication terminal, a wide dynamic range is required in the receiving system, and a gain control function is demanded in the low noise amplifier at the receiving front end. Besides, in the large-scale integration trend of the circuit, signal branching is indispensable in an integrated circuit.
Also in mobile communication, double side phase signals, it is advantageous to use double side phase signals because the incoming noise applied simultaneously in signal lines is eliminated. Thus, in the integrated circuit, this constitution is widely employed not only in the amplifier but also in the multiplier, oscillator and others. However, when a filter corresponding to double side phase signals is formed in the integrated circuit, the required area for inductor and capacitor is twice as wide as in the single side phase composition. Accordingly a circuit for converting a double side phase signal into a single side phase output is also needed. As the single side phase converter, it is general to connect the buffer of emitter follower type to the single side phase output of the differential amplifier.
A conventional variable gain amplifier is described below.
In a circuit diagram of a conventional variable gain amplifier in FIG. 13, a cascade amplifier is composed of transistors 1301, 1302, resistors 1303, 1304, inductor 1305, grounding capacitor 1306, input matching circuit 1307, output matching circuit 1308, gain control terminal 1309, power source terminals 1310, 1311, radio frequency input terminal 1312, and radio frequency output terminal 1313.
In this amplifier, in the case of weak input electric field, voltage Vagc1 for obtaining a high gain is applied to the gain control terminal 1309. At this time, the circuit composition of input matching circuit 1307 and output matching circuit 1308 and the inductance of inductor 1305 is set so that the input and output impedance of the amplifier coincide with the characteristic impedance of the transmission line. On the other hand, in the case of strong input electric field, voltage Vagc2 for low fain (Vagc2&lt;Vagc1) is set in the gain control terminal 1309 so that the gain of the amplifier is decreased.
In the conventional constitution of the variable gain amplifier, however, when changing over the gain, the input impedance of the amplifier varies. Then the input impedance mismatches with the characteristic impedance of the transmission line. Moreover since the gain is decreased by lowering the current of transistor, higher order distortion due to nonlinear effect increases.
In a conventional signal branching method, two stages of amplifiers are prepared, and the signal is branched off by the output of the first stage amplifier, and the signal is amplified again by the second stage amplifier, thereby compensating for the loss due to branching.
Besides, in the conventional signal branching constitution, to branch the signal into a plural lines, a large current must be passed in the first stage amplifier.
A conventional single side phase converter is described.
FIG. 14 is a circuit diagram showing a conventional constitution of single side phase converter. In FIG. 14, a differential amplifier 700 is composed of a direct-current voltage source 704, transistors 707 and 708 sharing a common emitter terminal, a load resistor 706 connected to the collector side of the transistor 708, and a current source 709 connected to the common emitter terminal of the transistors 707, 708. An emitter follower 1100 is composed of a transistor 1101 and a direct-current current source 1102 connected to the emitter of the transistor 1101. In addition, FIG. 14 further shows a common mode signal input terminal 702, an inverting phase signal input terminal 703 for receiving a signal differing in phase by 180 degrees from that of the common mode signal input terminal 702, and an output terminal 712.
In thus constituted single side phase converter, the operation is described below.
First, double side phase signals input from the common mode signal input terminal 702 and inverting phase signal input terminal 703 are amplified by the differential amplifier 700. At this time, a negative phase output signal appears at the collector terminal of the transistor 707, and a positive phase output signal appears at the collector terminal of the transistor 708. By picking up this positive phase output signal through the load resistor 706, the double side phase signal can be converted into a single side phase signal. It is a common practice to omit the load resistor of the transistor 707 which does not pick up signal. Consequently, this positive phase output signal is taken out after converting the impedance in the emitter follower 1100 which is a non-inverting amplifier.
In this constitutions of the single side phase converter, since only the positive phase output of the double side phase output is taken out, half of the consumption current is not used. Another problem is that it is not possible to eliminate the in-phase noise which is an advantage of double side phase driving.